Nowadays, color filters for use in a liquid crystal display or the like are each fabricated mainly by applying a colored composition, which is formed of a photoresist and a pigment dispersed therein, onto a substrate by a spin-coating method, coating method or transfer method, exposing the thus-applied colored composition to light, radiation or the like through a photomask, and then performing development to form pixels. Upon fabrication of such color filters, a diketopyrrolopyrrole pigment is generally used as a pigment for the formation of red pixels. Mere dispersion of this pigment together with a resin, a solvent and the like by a conventional disperser, however, cannot bring the pigment into a fully-dispersed form. When the thus-prepared dispersion is used for the formation of color pixels for the color filters (hereinafter simply called “color pixels”), the resulting red pixels lack transparency and have insufficient transmission as pixels for color filters. The red pigment is, therefore, dissatisfactory as a pigment for use in a colored composition for color filters. It is to be noted that the dispersion may mean a “colored composition for color filters” in some instances.
As resins for use in photoresists, acrylic polymers of high acid value are mainly adopted because an aqueous alkali solution can be used as a patterning developer. In a pigment dispersion composed of the above-described pigment and an acrylic resin of high acid value, however, the particles of the pigment undergo flocculation, so that the viscosity of the pigment dispersion tends to rise. Further, the pigment dispersion becomes thicker with time, and therefore, has poor storage stability in many instances.
When fabricating a color filter with such a pigment dispersion as accompanied by such difficulties as described above, the pigment dispersion is applied onto a substrate by a spin-coating method to form a colored film, and the colored film is then formed into pixels. When the viscosity of the pigment dispersion is high or when the pigment particles flocculate and the colored composition of color filters shows thixotropic viscosity, the coated layer rises at a central part thereof prior to its formation into pixels. When a large-size color filter is fabricated, this rise becomes a cause for the occurrence of differences in hue and density between the pixels formed at the central part and the pixels formed at the peripheral part of the substrate.
A colored composition for color filters (pigment dispersion) generally has a pigment concentration in a high range of from 5 to 20 wt. %. Nonetheless, the colored composition must be in a state dispersed without flocculation of pigment particles themselves, must have a viscosity (e.g., 5 to 20 mPa·s or so) lower than general room-temperature drying coating formulations or baking coating formulations, and must also be excellent in storage stability.
To meet the above-described requirements, it has conventionally been proposed that, when a red pigment is a diketopyrrolopyrrole pigment (for example, C.I. Pigment Red 254), one or more diketopyrrolopyrrole sulfonic acids, each having a degree of sulfonation of 1 or higher, be added to the pigment or the pigment be treated with the sulfonated product (JP-A-2000-160084).
On the other hand, the utility of liquid crystal displays has expanded from monitors for personal computers to color displays in color television sets, leading to a demand for further improvements in the performance of color filters. A need has, therefore, arisen for an improvement in the transparency of pixels, an increase in the contrast of light transmitted through pixels, and an increase in the concentration of pigment in pixels.
With the above-described methods which make use of the sulfonated product, however, it is difficult to improve the transparency of pixels by relying upon an improvement in the dispersibility of the pigment or to avoid an increase in the viscosity of the pigment dispersion or a reduction in its storage stability due to a rise in the pigment concentration. Further, it is difficult to control the reaction upon sulfonating the diketopyrrolopyrrole pigment. When a sulfonated product with many sulfonic groups introduced therein is used as a dispersant, foreign matter may occur in the resulting pigment dispersion. There is also an outstanding desire for improvements in these respects.